The present invention relates to lasers, and more particularly, to an improved Vertical Cavity Surface-Emitting Laser.
Light emitting devices, such as lasers, are being increasingly used in a variety of applications such as communications and data storage devices. One type of laser is the vertical cavity surface emission laser (VCSEL). FIG. 1 illustrates a cutaway side view of a sample prior art VCSEL 10. The sample VCSEL 10 is a conventional oxide-confined top emitting VCSEL 10. The VCSEL 10 includes a top reflector 20 and a bottom reflector 30 sandwiching an active region 40. The reflectors 20 and 30 and the active region 40 are fabricated on a substrate 50. A bottom electrode 52 is connected to the substrate while a top electrode 54 is connected to the top reflector 20.
The substrate 50 may be n-type gallium arsenide (GaAs) doped with silicon. The bottom electrode 52 forms an ohmic contact to the substrate 50 and is typically made of electrically conductive metal such as Gold-Germanium(AuGe) alloy.
The active region 40 includes a light generation layer 42 which is typically constructed from one or more quantum wells of InGaAs (indium gallium arsenide), GaAs (gallium arsenide), AlGaAs (aluminum gallium arsenide), or InAlGaAs (indium aluminum gallium arsenide). The light generation layer 42 is separated from the top reflector 20 by a top spacer 44 and separated from the bottom reflector 30 by a bottom spacer 46. The light generation layer 42 is configured to generate light having a predetermined or known wavelength. The predetermined wavelength can vary widely, for example, within a range from approximately 650 nanometers (nm) to approximately 1,400 nanometers. The spacer layers 44 and 46 may be fabricated using, for example, AlGaAs.
The active region 40 is sandwiched between the top reflector 20 and the bottom reflector 30. The top reflector 20 and the bottom reflector 30 are distributed Bragg reflectors (DBR) including alternating quarter wavelength thick layers of materials having differing optical index of refraction such as, for example, AlAs, GaAs, or AlGaAs having differing ratios of Aluminum and Gallium. For this reason, the top and bottom reflectors 20 and 30 are also referred to as DBR mirrors 20 and 30.
Conventionally, each layer of the DBR mirrors 20 and 30 has a thickness that is one-fourth ( 1/4) wavelength of the light generated by the active region 40. For simplicity, only eight layers (four pairs of layers) are illustrated in FIG. 1 for each of the DBR mirrors 20 and 30. In actual implementations each of the DBR mirrors 20 and 30 may include many more layers such as twenty or thirty pairs of layers.
In order to create the optical gain required for a semiconductor laser to operate, the active region 40 is composed of a p-n junction and electrical current is injected into it. To facilitate the flow of electrical current through the VCSEL 10, the DBR mirrors 20 and 30 are doped with carefully designed profiles of n- or p-type dopants in order to both improve the bulk material conductivity and to minimize the voltage drop across the many heterobarriers formed at the interfaces between the alternating quarter-wave layers.
While the heat generated due to bulk resistance and heterobarrier voltage drop in the conducting DBR mirrors can be mitigated by careful design of the doping profile, the heat generated in the p-n junction of the active region is intrinsic to the operation of the device and can not be fully eliminated. Normally, one DBR mirror is doped n-type and the other doped p-type, forming the p-n junction and its associated heat dissipation in the active region 40. For example, the top DBR mirror 20 may include p-doped GaAs, AlAs, or AlGaAs layers while the bottom DBR mirror 30 include n-doped GaAs, AlAs, or AlGaAs layers. The n dopant may be silicon and the p dopant may be carbon.
In the present sample VCSEL 10, the layers of the top reflector 20 are doped as p-type semiconductors and the layers of the bottom reflector 30 are doped as n-type semiconductors. The substrate 50 is doped to create an n-type contact.
The VCSEL 10 of such structure and its operations are known in the art. To produce laser light (lasing effect), electric current is introduced to the VCSEL 10 via the electrodes 52 and 54. When the current flows through the active region 40, photons (light particles) are generated by the quantum wells of the light generation layer 42. With sufficient current through the active region 40, optical gain is created that coherently amplifies the light which reflects back and forth between the DBR mirrors 20 and 30. A portion of the light is transmitted through the DBR mirrors 20 and 30, and an opening 56 in the top electrical contact is typically employed to allow the transmitted light out of the device 10. This escaping light is indicated by arrow 58.
Current confinement barrier 60 is often used to direct the electrical current generally toward the middle of the active region 40. When used, the current confinement barrier 60 insulates all but a circular or polygon-shaped area (from a top perspective, not shown) having a diameter that is typically similar to or smaller than the contact opening width 57. Because most of the electrical current is directed toward a portion 43 of the light generation layer 42, most of the light is generated within this portion 43 referred to as the active portion 43 herein.
To generate more light from the VCSEL 10, more current is applied to the VSCEL 10. Increased current not only results in more light but also in more heat generated at the active region 40. The heat adversely impacts the VCSEL 10 limiting the amount of light that can be generated by the VSCEL 10.
Excess heat in a VCSEL has a number of adverse effects including wavelength shift, gain reduction, increased absorption, refractive index shift and associated optical loss and DBR mirror reflectivity changes, leakage current, thermal lensing, and reduced efficiency. Combination of these effects and their spatial variation ultimately limits the maximum power, maximum single-lateral-mode power, maximum modulation bandwidth, and maximum efficiency the VCSEL 10 can achieve. Further, all of these parameters are of commercial importance. For example, long-wavelength (1300 nm wavelength of emitted light) VCSELs could be commercially lucrative as light sources in optical communication links running at very high bit rates over metro-area link lengths of 10 kilometers (km) or more, but heat generation in the VCSELs makes it very challenging to simultaneously meet the modulation speed, optical power, and single-mode optical power requirements of such links.
FIG. 2 illustrates a lateral temperature distribution curve 70 along the active region 40 similar to results obtained by detailed finite element modeling. Here, layer interval 42i represents and corresponds to the extent of the light generation layer 42 of the VCSEL 10 of FIG. 1 and active interval 43i represents and corresponds to the extent of the active portion 43 of the light generation layer 42 where most of the photons are generated. As illustrated in FIG. 2, temperature is highest at the active interval 43i and the temperature drops off sharply at the edges of the active interval 43i. Temperature differential 72 between the temperature at the active interval 43i and the temperature outside the active interval 43i is relatively large. The lateral temperature distribution curve 70 suggests that heat is effectively trapped within the active portion 43 of the VCSEL 10 and is not effectively distributed or dissipated thereby exacerbating the heat problems of the VCSEL 10 of FIG. 1.
The heat is trapped within active portion 43 largely because of the normally conflicting requirements of high optical contrast, high electrical conductivity, and high thermal conductivity of the DBR mirrors 20 and 30. DBR mirrors are typically designed for high optical contrast and low electrical loss. Further, DBR mirrors normally have relatively poor thermal conductivity resulting in heat generated in active portion 43 being trapped and forming high peak temperatures as illustrated in curve 70 of FIG. 2. The high peak temperature not only exacerbates problems associated with overheating such as limited efficiency, but the narrow spatial distribution caused by the heat confinement exacerbates problems associated with the lateral heat distribution such as thermal lensing.
Accordingly, there remains a need for an improved laser with improved heat dissipation characteristics thus alleviating or eliminating the adverse effects that heat has on the light emitting device.